Deleting NDF from file group completely?

A file can be removed from the database only if the file is empty. Without SHRINKFILE, the implication is that the file must be the only file in a user-defined filegroup and you must first drop or move all the objects (or partitions) from the filegroup to a different file group. The empty file can then be dropped with ALTER DATABASE...REMOVE FILE.

It seems your objective is to delete data older than 6 months. It would be easier to just delete/truncate the data and not bother with files/filegroups at all.